Memory leak in scanPlanString

**Describe the bug**
There is a memory leak while using pgx Select Scans. There is a consistent increase of memory usage by the package. Here is a pprof dump -
```
reflect.New:3256 github.com/jackc/pgx/v5/pgtype.scanPlanString - 4.65 GB
github.com/jackc/pgx/v5/pgtype.scanPlanString - 2.64 GB
reflect.New:3256 github.com/georgysavva/scany - 834.60 MB
*github.com/georgysavva/scany/v2/dbscan.(RowS) - 821.96 MB
reflect.Value.extendSlice:2848 reflect.Append - 816.12 MB
reflect.New:3256 github.com/georgysavva/scany - 794.36 MB
github.com.jackc/pgx/v5/pgtype.TextCodec.Deco - 568.66 MB
github.com.jackc/pgx/v5/pgtype.scanPlanString - 360.29 MB
github.com.jackc/pgx/v5/pgtype.scanPlanString - 320.38 MB
```

These are the top memory users in my application. The query I make is quite simple and is just made once every 30 minutes. The package does seem to be leaking the memory on every query though.

**To Reproduce**
Steps to reproduce the behavior:
You can probably just make a large query again and again and pprof the process to see the memory usage.

**Expected behavior**
Memory should only be used ephemerally by the package.

**Actual behavior**
Gigs of memory leak
**Version**
- Go: go 1.21
- PostgreSQL: pg 14

- pgx: github.com/jackc/pgx/v5 v5.5.5

**Additional context**
Add any other context about the problem here.

Start with pgtype.scanPlanString and the pgtype.TextCodec decode path named in the pprof output. Reproduce repeated large queries with Go 1.21 and pgx v5.5.5, then compare memory profiles to determine whether allocations remain retained after each query. Done means the reported growth is explained and a verified change prevents the gigabytes of retained memory.

Written by the indexing model from the issue text.